Corrosion resistant alloy

ABSTRACT

AN ESSENTIALLY MARTENSITIC CR-NI-MO(W)-CU STAINLESS STEEL WHICH HAS GOOD CORROSION RESISTANCE IN ACID MEDIA AND HIGH STRENGTH.

United States Patent 3,574,601 CORROSION RESISTANT ALLOY Lewis P. Myers, Mount Penn, and Kermit J. Goda, Jr.,

Leesport, Pa., assignors to Carpenter Technology Corporation, Reading, Pa. No Drawing. Filed Nov. 27, 1968, Ser. No. 779,609 Int. Cl. C22c 39/20 US. Cl. 75-125 4 Claims ABSTRACT OF THE DISCLOSURE An essentially martensitic CrNi-Mo [W]Cu stainless steel which has good corrosion resistance in acid media and high strength.

This invention relates to a stainless steel alloy and more particularly to a stainless steel having a unique combination of high strength and corrosion resistance but yet is relatively inexpensive.

In the chemical and petroleum industries there are many instances when the combination of strength and corrosion resistance required cannot be satisfied by currently available alloys without recourse to alloys that are so expensive that their use becomes prohibitive. To our knowledge, at the present time a usable combination of high strength with good corrosion resistance to oxidizing and reducing acid media is available only with alloys such as A.I.S.I. type 660 which are very expensive compared to the 300 and 400 series alloys. Of the more moderately priced alloys, A.I.S.I. type 410 can have a minimum .2% yield strength of about 100,000 p.s.i. but has virtually no resistance to attack by acids. Though only a little more costly than type 410, A.I.S.I. type 304 has excellent resistance to an oxidizing acid such as nitric acid, but unfortunately its less than 50,000 p.s.i. .2% yield strength makes it entirely unsuited for use where high strength and corrosion resistance are required. Even though A.I.S.I. type 316 is substantially more expensive than type 304 (about 20% or more), it has no better strength, but it does withstand attack by a reducing acid medium such as sulfuric acid.

It is, therefore, a principal object of this invention to provide a stainless steel alloy which in its annealed condition has a room temperature .2% yield strength of about 95,000 p.s.i. or more, loses no more than 100 mils per year (m.p.y.) and preferably no more than 50 m.p.y. in boiling 65% by weight nitric acid, has good resistance to corrosion in 5% by weight sulfuric acid at room temperature, and which has good resistance to hydrogen embrittlement.

Another object of our invention is to provide such an alloy which when cooled from its annealing temperature has the desired strength and corrosion resistance, and also is sufficiently soft and ductile so that it can easily be formed and machined.

Further objects as well as advantages of the present invention will be apparent from the following detailed description and illustrative examples thereof. I

We have found that the composition capable of producing the aforementioned results is as follows in approximate percent by weight in accordance with good metallurgical practice, but only when balanced to provide an essentially martensitic microstructure containing no more than ice about 10% retained austenite and no more than 1 to 2% delta ferrite:

Broad range Preferred range Carbon 0.2 max.-. Manganese 0.1 max,

0 Silicon Chromium- Nickel 1 For free machining properties as much as 3.5% manganese and/or as much as 0.5% sulfur can be included. For this purpose, selenium on a 1 for 1 basis can be substituted for all or part of the sulfur.

2 Or an equivalent amount of tungsten.

The remainder of our composition is preferably essentially iron except for impurities incidental to good commercial metallurgical practice. When our alloy is prepared using, as for example, an electric arc furnace, deoxidizers in addition to or in place of manganese and silicon, such as aluminum, zirconium, magnesium, or the rare earths may be used with the result that a small amount from about .01% to 1% of one or more of these elements maybe present as an incidental impurity. Nitrogen normally may be present as an impurity, but because it is a powerful austenite former (about 30 times as effective as an equal amount in weight percent of nickel) and also because of its hardening effect, it is preferably not added and should not be present in an amount greater than 0.1%. Additional elements may be present in our composition which do not detract from its desired properties.

The elements chromium, nickel, molybdenum (or an equivalent amount of tungsten) and copper are the only essential alloying elements in our steel. When carefully balanced within the ranges stated so as to ensure an essentially martensitic microstructure, they provide the corrosion resistance characteristic of our alloy and the martensitic reaction provides its strength. That is why in balancing the alloy, the effect on the microstructure of all the elements present is taken into account whether added intentionally or not. The presence of austenite does not usually detract from corrosion resistance, but when the amount of retained austenite is greater than about 10%, the martensite reaction is no longer capable of consistently providing the desired strength. On the other hand, the presence of small amounts of delta ferrite does not adversely affect the strength of our alloy, but more than as little as 1 to 2% results in a significant loss of resistance to attack by acid. Thus, delta ferrite should be kept to a minimum and no more than 1 to 2% should be present in our alloy. For best results the alloy should be entirely free of delta ferrite.

Carbon, like nitrogen, is a powerful austenite former, and unless stabilized, preferably by an appropriate amount of columbium or titanium, should also be considered as 30 times as effective as nickel. Carbon is usually not an intentional addition to our composition, but is considered tolerable in amounts of up to a maximum of about 0.2%. Preferably, carbon is held to no more than about 011% and when present in an amount above about 0.03%, should be accompanied by an amount of columbium in weight percent equal to about 10 times its weight or by an amount of titanium equal to about 5 times its weight percent. Both columbium and titanium in the proportions stated work to stabilize the carbon so that it is no longer effective to form austenite. Other carbon stabilizers that can be used include zirconium, vanadium and tantalum in a total amount up to 1% either individually or together, from to times the carbon content being used. Unless the carbon is stabilized, its weight percent should be multiplied by 30 in calculating nickel equivalent when balancing our composition. Also, unless stabilized the carbon present in our composition tends to harden the martensite that is formed as the alloy is quenched from its annealing temperature. This hardening can offset to some extent the weakening offset which usually accompanies the retention of more than 10% austenite, but the hardening effect is not desired when it interferes with fabrication of the alloy. When this hardening effect is not considered objectionable and the proportions of the remaining elements are such that the carbon need not be stabilized to ensure the martensitic balance of the alloy, then the addition of carbon stabilizers can be dispensed with.

Neither manganese nor silicon is a desired alloying addition in our alloy although manganese is tolerable up to a maximum of about 2%, and where free machining properties are preferred over some loss in corrosion resistance in oxidizing acids, then manganese can be increased up to about 3.5%. Manganese is about half as effective as nickel as an austenite former. Therefore, in calculating the nickel equivalent, the manganese content should be multiplied by 0.5. Silicon does not appear to add to or detract from the chemical and physical properties of our alloy except that it can add to the capability of our alloy to resist oxidation. However, so long as it does not adversely affect the martensitic balance of our alloy, silicon can be present in an amount ranging up to a maximum of about 2.5%. Silicon as a ferrite former is 1.5 times as effective as chromium. Thus in calculating the chromium equivalent content of our alloy, the silicon content in weight percent should be multiplied by 1.5.

Phosphorus and sulfur are each limited to a maximum of no more than about 0.05%, preferably 0.03%, except that when free machining properties are wanted and the accompanying reduction in corrosion resistance is tolerable, then as much as 0.5% sulfur can be added. Also, selenium can be used instead of all or part of the sulfur to provide the desired free machining properties.

For its effect in providing stainless properties and the required maximum loss of less than 100 m.p.y. in an oxidizing acid such as nitric acid, a minimum of about 13.5% chromium must be included in our alloy. Chromium being a ferrite forming element, the maximum amount of chromium that is tolerable in our composition is therefore determined by this effect and is about 17%. Because of its effect on the oxidizing acid resistance of the composition, we preferably include at least about 14.75% chromium to ensure a maximum loss of metal in boiling 65% by weight nitric acid of less than 50 m.p.y., and better yet with a minimum of chromium the maximum rate at which metal is lost from our composition in boiling nitric acid is less than 30 m.p.y. Because of the ferrite forming efiect of chromium, we prefer to limit the maximum amount of chromium to no more than about 16.25% but, as will be more fully pointed out hereinbelow, care must be exercised at each level of nickel and chromium to ensure that the martensitic balance of the alloy is preserved.

Nickel is essential to the corrosion resistance of our composition, and to this end a minimum of 4% nickel is required. Nickel also works to balance the microstructure and is an austenite former. For these purposes 4 to 9% and preferably 5 to 7% nickel is included in our composition. When balancing the alloy, the larger amounts of nickel or nickel equivalent go with the smaller amounts of chromium or chromium equivalent in order to avoid the presence of more than 10% retai ed austenite. While 4 the corrosion-resistance effect of nickel in our composition provided by the required minimum of 4% finds no equivalent in other elements, other austenite-forming elements can be used in place of the nickel in excess of about 4% so long as the desired properties are not adversely affected. Thus manganese is not a desirable substitute, but cobalt is as will be more fully pointed out.

Molybdenum, like chromium, is a ferrite-forming element, and its effect is such that in calculating the chromium equivalent content of our composition, 1% of molybdenum should be considered as 1% of chromium. Molybdenum adds to the corrosion resistance of our alloy and is most important in ensuring the desired resistance to a reducing acid environment such as sulfuric acid. For this purpose, a minimum of about 0.5% molybdenum is required. Molybdenum is also effective against hydrogen embrittlement, particularly when products formed of our alloy are to be exposed to a hydrogen-bearing environment in an aged condition. Molybdenum-rich delta ferrite seems to be even more readily attacked than chromium-rich delta ferrite, and thus care must be exercised to ensure that for a given balance there is not so much molybdenum present that it causes the formation of ferrite. Therefore, we limit molybdenum to 3% in our composition. For best results, we preferably use 0.75 to 2% molybdenum.

Molybdenum and tungsten can be used interchangeably in our alloy. To replace molybdenum with an equivalent effect, tungsten in the proportion of about 1.2% to 1.6% tungsten for 1% molybdenum can be substituted for all or part of the molybdenum content of our alloy. It is therefore to be understood when molybdenum is referred to in this application it is intended to include molybdenum and tungsten either together or individually with the tungsten replacing all or part of the molybdenum in the proportion stated.

Copper and molybdenum (or the equivalent amount of tungsten) work together in our composition to provide enhanced resistance to attack by sulfuric acid. For example, we have found that 1.5% copper with 0.75% molybdenum provides an equal or greater effect than as much as either 3% copper or 3% molybdenum in the absence of the other. Copper also assists in providing the required corrosion resistance particularly resistance to attack by reducing acid media such as sulfuric acid. Copper also provides a hardening effect when the alloy in its annealed condition is too soft or greter strength is required. When present in amounts less than about 0.75 copper is not effective and the corrosion resistance of our alloy cannot be attained. Above about 3% further additions of copper can have a harmful effect on the alloy. We prefer to limit copper to about 2% for best results. As an austenite former, copper is one half as effective as nickel, and thus the Weight percent of copper is multiplied by .5 in calculating the nickel equivalent content of our alloy.

As was seen, cobalt can be substituted for some but not all of the nickel content in our alloy. Up to about 6% cobalt can be used in the proportion of 3% cobalt for each 1% nickel of the nickel in excess of 4% in our alloy. Cobalt is about one-third as effective as nickel in our alloy as an austenite former and thus provides a way of increasing the alloy content without over balancing the composition. When cobalt is present in our composition, only one-third of its weight percent is used in calculating the nickel equivalent content of our composition.

Because of the effect it can have on the elevated temperature properties of our alloy, up to about 0.01% boron. can be included in our alloy, but when best corrosion resistance is desired to acid media such as nitric acid which attack the grain boundaries, then boron should not be intentionally added.

By way of summary it may be noted that nickel, copper,

carbon, nitrogen and manganese act as austenite formers. Their relative effect is approximately indicated as:

But carbon tied up by a stabilizer is not to be counted as an austenite former. The elements chromium, molybdenum and silicon tend to form delta ferrite in our composition. Their relative effect is approximately indicated as:

The elements that form ferrite also tend to stabilize austenite. Thus, for a given amount of nickel equivalent the amount of retained austenite may increase with increasing chromium equivalent content in the alloy.

From the foregoing it is seen that the minimum nickel equivalent content possible in our composition is about 4.37% and the minimum chromium equivalent content possible is about 14%. That relationship provides a fully martensitic microstructure. As a guide in balancing our alloy, it may be noted that at the nickel equivalent level of 4.37%, our data indicats that up to about 16 chromium equivalent can be included without leading to more than 1 to 2% delta ferrite. As the chromium equivalent content in our alloy is increased from about 16% to about 18.5%, the nickel equivalent content should be increased from 4.37 to about 8.6%. Although the relationship may not be linear, it is useful to treat it as such as a guide in balancing the alloy. It should also be noted that at the 14% chromium equivalent level, the nickel equivalent content of our alloy may range up to about 10% or one or two-tenths of a percent less. As the chromium equivalent content is increased from 14% to about 1 8.5% the nickel equivalent content must be decreased from about 10% by somewhat more than 1% to about 8.6%. This also is not a strictly linear relationship but it is useful to treat it as such as a guide in balancing the alloy so as to avoid the presence of more than about 10% retained austenite.

Our alloy is readily prepared and worked in accordance with good standard commercial practice. Heat treatment is not critical, and it can be annealed by heating at about 1700 to 2100 F. for about one-half hour to one hour. We prefer to anneal at about 1800 to 1950 F. When greater strength than the minimum of about 100,000 p.s.i. .2% yield strength is required, our alloy can be aged by heating at from about 800 to 1000" F. for about 2 to 8 hours.

The following examples of our alloy were melted and cast into experimental ingots which were forged, annealed and formed into test specimens. Unless otherwise indicated, annealing was carried out at 1800 F. for one-half hour and was followed by quenching in water. Room temperature tensile property specimens had a 0.252-inch gage diameter and a one-inch gage length. Corrosion-resistance specimens were 1% ins. x /2 in. X /s in. which were carefully weighed to within 0.0001 gram before. and after exposure to the test environment and the corrosion rate in mils per year (m.p.y.) was calculated. The hydrogen embrittlement specimens used were 8-inch long bars having a 0.25 in. diameter with a 0.125 in. gage diameter. In all examples, phosphorus and sulfur were less than 0.01%.

EXAMPLE 1 As a specific example of our composition, a heat was melted and an ingot cast containing in percent by Weight:

6 Carbon 0.010 Manganese 0.38 Silicon 0.32 Chromium 14.83 Nickel 5.77 Molybdenum 1.49 Copper 0.75 Nitrogen 0.023

Iron, remainder except for incidental impurities.

In its annealed condition the alloy had a hardness of Rockwell F 31 and room temperature tensile properties of about 113,500- p.s.i. .2% yield strength, an ultimate tensile strength of about 143,000 p.s.i. with an elongation of about 16.5% and a reduction of area of about 63%. To test its resistance to hydrogen embrittlement, two specimens were subjected to a stress of 80,000 p.s.i. in a solution of 5% by weight acetic acid saturated with hydrogen sulfide at room temperature. After 263 hours neither specimen had failed, and the test was discontinued. Two additional specimens were tested in the same hydrogenbearing environment after being annealed, then aged for 4 hours at 850 F. and air cooled. After 289 hours under a stress of 80,000 p.s.i. neither specimen had failed, and the test was discontinued. After being tested for 5 48-hour periods in boiling 65% by weight nitric acid the average rate at which metal was lost was calculated and found to be about 33 to 34 m.p.y. Test specimens were also immersed in test solutions of 5% by weight sulfuric acid at room temperature and no metal was lost. X-ray diffraction analysis of the microstructure of the alloy indicated no retained austenite and the alloy had no delta ferrite.

EXAMPLE 2 As another specific example of our alloy, a heat was melted and an ingot cast containing in percent by weight:

Carbon o 0.0 16

Manganese 0.30 Silicon 0.26

Chromium 15.58 Nickel 5.94 Copper 1.42 Molybdenum 0.80 Columbium 0.16 Nitrogen 0.024

Iron, remainder except for incidental impurities.

In its annealed condition, the alloy had a hardness of Rockwell C 29 and room temperature tensile properties of about 113,500 p.s.i. .2% yield strength, an ultimate tensile strength of about 148,500 p.s.i. with an elongation of about 16.5% and a reduction in area of about 67%. Test specimens of this alloy were immersed in 65% by weight boiling nitric acid, and after 5 48-hour periods, the rate at which metal was lost was calculated and was found to average about 26 to 27 m.p.y. Four test specimens of this alloy were also immersed in 5% by weight sulfuric acid at 50 C., and subjected to 3 48-hour periods. Then the rate at which metal was lost was calculated and was found to average 86 m.p.y. for 2 test specimens and 111 and 116 m.p.y. for 2 other test specimens. In 5% by weight sulfuric acid at room temperature test specimens lost no metal. As a test for hydrogen embrittlement, test specimens were subjected to a stress of 80,000 p.s.i. in 5% by weight acetic acid saturated with hydrogen sulfide at room temperature. After 260 hours, the specimens did not fail indicating the alloy was not sensitive to hydrogen embrittlement and the test was discontinued. X-ray diffraction analysis of the microstructure of the alloy in its annealed condition showed that it had about 3% retained austenite.

7 EXAMPLE 3 As another specific example of our composition, a heat was melted and an ingot cast containing in percent by weight:

Carbon 0.062 Manganese 0.33 Chromium 15.60 Nickel 5.87 Copper 1.40 Molybdenum 0.80 Columbium 0.53

Iron, remainder except for incidental impurities.

In this example, the elements silicon and nitrogen were substantially the same as in Example 2.

In its annealed condition, this alloy had a hardness of Rockwell C 32, a .2% yield of about 120,500 p.s.i. and an ultimate tensile strength of about 152,500 p.s.i. with an elongation of about 18.5% and a reduction in area of about 65.5%. Test specimens of this alloy were immersed in test solutions of 65% by weight boiling nitric acid. After 5 48-hour periods, the average rate at which metal was lost was calculated and was found to be 24 m.p.y. Test specimens of this alloy were immersed in 5% by weight sulfuric acid at a temperature of 50 C. After 3 48-hour periods, the average rate at which metal was lost was calculated and was found to be 0.1 m.p.y. When subjected to a stress of 80,000 p.s.i. in 5% by weight acetic acid at room temperature saturated with hydrogen sulfide, failure did not occur after 260 hours, showing that the alloy was not sensitive to hydrogen embrittlement and the test was discontinued. The microstructure was subjected to X-ray diffraction analysis and was found to contain 8% retained austenite.

Because of its outstanding resistance to attack by acids such as boiling 65% by weight nitric acid and 5% by weight sulfuric acid both at room temperature and at a temperature as high as 50 C. or higher combined with its high strength and freedom from hydrogen embrittlement, our alloy is especially Well suited for use in fabricating parts to be used in the chemical and petroleum industries.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. A corrosion resistant martensite stainless steel alloy having good resistance to corrosion in nitric acid and sulfuric acid and to hydrogen embrittlement, consisting essentially in weight percent of about Carbon-02 maximum Manganese3.5 maximum Silicon-2.5 maximum Phosphorus-005 maximum Sulfur-0.5 maximum Selenium-0.5 maximum Chromium-13.547 Nickel-4-9 Molybdenum-053 8 Copper0.753 Columbium-up to 10X %C Titaniumup to 5 x0 Cobalt-6 maximum Boron0.01 maximum Zirconium Vanadium up to 1 Tantalum J the remainder being essentially iron and incidental impurities, in which tungsten in the proportion of about 1.2 to 1. 6% tungsten to 1% molybdenum is the equivalent of and can replace all or part of the molybdenum, in which the percent nickel equivalent content calculated as is no less than about 4.37% and no more than about 10%, the percent chromium equivalent content calculated as is no less than about 14% and no more than about 18.5%, the amount of nickel equivalent and of chromium equivalent in said alloy being such that with a nickel equivalent content of about 4.37% the chromium equivalent content is no more than about 16%, as the chromium equivalent content ranges from about 16% to about 18.5 the nickel equivalent content ranges up to about 8.6%, with a chromium equivalent content of about 14% the nickel equivalent content is no more than about 10% and the nickel equivalent ranges from a maximum of about 10% to no more than about 8.6% as the chromium equivalent content ranges from about 14% to about 18.5 whereby said alloy in its annealed and quenched condition has an essentially martensitic microstructure containing no more than about 1 to 2% delta ferrite and no more than about 10% austenite, and has a .2% yield strength at room temperature of at least about 95,000 p.s.i.

2. The alloy as set forth in claim 1 which contains no more than about 2% manganese, and no more than about 0.05% sulfur.

3. The alloy as set forth in claim 1 which contains no more than about 0.1% carbon, no more than about 0.75% manganese, no more than about 0.5% silicon, no more than about 0.03% phosphorus, no more than about 0.03% sulfur, about 14.75 to 16.25% chromium, about 5 to 7% nickel, about 0.75 to 2% molybdenum, about 1 to 2% copper, columbium in an amount up to 10 times the percent carbon, titanium in an amount up to 5 times the percent carbon, and the remainder essentially iron.

4. The alloy as set forth in claim 3 which contains at least about 15% chromium.

References Cited UNITED STATES PATENTS 2,687,955 8/1954 Bloom l25 3,152,934 10/1964 Lulq 75--l25UXP 3,357,868 12/1967 Tanczyr 75125X 3,401,036 9/1968 Dulis 75125X 3,408,178 l0/l968 Myers 75-l25 HYLAND BIZOT, Primary Examiner Patent No.

Inventor(s) UNITED STATES PATENT OFFICE Dated April 13 1971 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column Column Column Column Column Column (SEAL) Attest:

MCCOY M. GIBSON, JR. Attesting Officer 13, for "offset" read effect 48, for "greter" read greater 28, for "indicate" read indicates 11, for "F" read C 17, after "yield" insert strength 51, for "martensite" read martensitic 3, after "X" insert v 59, References Cited, for "Lulq" read Lula 60, References Cited, for "Tanczyr" read Tanczyn Signed and sealed this 16th day of July 1974.

C. MARSHALL DANN Commissioner of Patents 

